Insulated containers such as semi-trailer bodies, railcars truck bodies and shipping containers are used to transport various perishables in refrigerated or frozen condition and most of these containers, when comprising a vehicle body portion, are equipped with mechanical refrigeration units to provide the desired cooling of the container interiors.
However, mechanical refrigeration units are subject to failure and shipping containers transported by ship or rail cannot be conveniently and economically mechanically refrigerated. Accordingly, a recent trend has developed in the refrigerated container field to utilize liquid CO.sub.2 as a means of maintaining the interiors of refrigerated containers at the desired temperatures.
When CO.sub.2 is used in this manner a tank is conventionally provided within the container to be chilled and liquid CO.sub.2 is sprayed into the tank in order to form CO.sub.2 snow therein. The tank is usually vented to the exterior of the associated container and constructed of good heat transfer materials, whereby the atmosphere within the container may be passed in good heat transfer relation with the external surfaces of the tank in order to achieve the desired cooling effect whenever the temperature within the container rises above a predetermined high temperature. This type of refrigeration has proven to be adequate, inexpensive, trouble free and reliable. However, in some instances wherein a CO.sub.2 cooled container is to be maintained chilled over extended periods of time and therefore must be provided with a reasonably large quantity of CO.sub.2 snow in order to provide the desired cooling over that extended period of time, the cooling effect of the large quantity of CO.sub.2 snow within the container, even independent of mechanically assisted airflow over the outer surfaces of the CO.sub.2 tank, causes the temperature within the container to drop below the desired minimum temperature within the container. This causes produce which is being shipped to become frozen and spoiled. Accordingly, a need exists whereby reasonably large quantities of CO.sub.2 snow contained within a refrigerated container may be prevented from excessively cooling the interior of the container. Still further, a need also exists for the same container to also be usuable in transporting frozen foodstuffs.